libfetchers/github: allow url attribute

[Ma27]

Since 108debe we allow a
url-attribute for the github-fetcher to fetch tarballs from
self-hosted gitlab/github instances.

However it's not used when defining e.g. a flake-input

foobar = {
    type = "github";
    url = "gitlab.myserver";
    /* ... */
}

and breaks with an evaluation-error:

error: --- Error --------------------------------------nix
unsupported input attribute 'url'
(use '--show-trace' to show detailed location information)

This patch allows flake-inputs to be fetched from self-hosted instances
as well.

[edolstra]

url seems wrong since gitlab.server is not a valid URL. Maybe it should be server or host or something like that?

[Ma27]

@edolstra you're obviously right and I'm fine with a rename. I'd just like to point out that I used the name-decision that was originally taken in 108debe.

I think host is better. If you agree with this I'll update my PR accordingly.

[edolstra]

Ah, sorry about that. Yes please do!

[Ma27]

@edolstra fixed. Please note that this needs to be fixed as well now in https://github.com/edolstra/flake-compat/blob/master/default.nix :)